I've never found Darius to be op, just annoying. If you counter pick him and play smart, he becomes so overwhelmingly useless that the team that has him will typically be complaining about how bad their Darius is, even though there is quite literally nothing he can do to fight against it.
Champs like Kayle, or Nunu are so exceptionally strong against him that it's painful. Zilean is another guy who crushes him, and someone like Corki or Ezreal will never lose an engagement to him.
I play Katarina nearly exclusively nowadays and have had tremendous success vs Darius, and I like to think I play at a fairly high level (2100+ before jabe bot got booted).
I've said this a lot, but I'll say it again. Darius is an opportunist champion. If he gets his opportunity, he is ruthless and will crush entire teams almost by himself. However, if your team has good picks and you play smartly enough, he's borderline worthless. The only thing he really does is damage outside of a short ranged super long cd pull and slow, so he better be able to dish it out when he gets his chance.